With the growing adoption of Internet-capable smartphones, wireless carriers face competition from Over-the-Top (OTT) messaging providers such as Facebook™ Messenger, WhatsApp™, Kik™, Tango™, Viber™ and others. These OTT providers typically operate closed chat systems designed in such a way that OTT users and RCS messaging users on wireless carriers cannot form mixed group chat sessions. This is a method by which carriers on one chat system, such as wireless carriers using Rich Communication Suite, can invite and enable OTT messaging users to join an operator based Rich Communication Suite (RCS) based instant messaging session. Notably this method only requires that one member of an ongoing Rich Communication Suite (RCS) group chat session be a member of the OTT messaging service in order for all members of the RCS group to exchanges messages with the OTT message service. Conventional solutions have proposed to set-up a server-to-server cooperation (chat federation) between OTT messaging systems and RCS-based messaging systems or use of a gateway. These proposals work for scenarios where every RCS user has an account in both messaging system, but they create a poor user experience for group chat sessions involving users who have an account in only one of the messaging systems. What is needed is a method to easily invite members of an OTT message service into an ongoing RCS chat without every member of the RCS chat needing credentials in the OTT messaging system.